Why do frogs and butterflies lay more eggs at a time?

Because these are animals that we would classify as having a high mortality rate. Meaning, that the percentage of eggs that make it to adulthood is very low. They compensate for this by laying a lot more eggs at a time, ensuring that a greater number make it to adulthood. That is also why humans only have one offspring at a time, and rarely more. Because our mortality rate is so low, that it is not necessary for our species survival to have more than one.